How to use the aniso8601.duration.parse_duration function in aniso8601

#Is expressly not supported as there is no way to provide the addtional
    #required context.

    firstpart, secondpart = isointervalstr.split(intervaldelimiter)

    if firstpart[0] == 'P':
        #/
        #Notice that these are not returned 'in order' (earlier to later), this
        #is to maintain consistency with parsing / durations, as
        #well as making repeating interval code cleaner. Users who desire
        #durations to be in order can use the 'sorted' operator.

        #We need to figure out if  is a date, or a datetime
        if secondpart.find(datetimedelimiter) != -1:
            # is a datetime
            duration = parse_duration(firstpart, relative=relative)
            enddatetime = parse_datetime(secondpart, delimiter=datetimedelimiter)

            return (enddatetime, enddatetime - duration)
        else:
            # must just be a date
            duration = parse_duration(firstpart, relative=relative)
            enddate = parse_date(secondpart)

            #See if we need to upconvert to datetime to preserve resolution
            if firstpart.find(datetimedelimiter) != -1:
                return (enddate, datetime.combine(enddate, datetime.min.time()) - duration)
            else:
                return (enddate, enddate - duration)
    elif secondpart[0] == 'P':
        #/
        #We need to figure out if  is a date, or a datetime

if firstpart.find(datetimedelimiter) != -1:
                return (enddate, datetime.combine(enddate, datetime.min.time()) - duration)
            else:
                return (enddate, enddate - duration)
    elif secondpart[0] == 'P':
        #/
        #We need to figure out if  is a date, or a datetime
        if firstpart.find(datetimedelimiter) != -1:
            # is a datetime
            duration = parse_duration(secondpart, relative=relative)
            startdatetime = parse_datetime(firstpart, delimiter=datetimedelimiter)

            return (startdatetime, startdatetime + duration)
        else:
            # must just be a date
            duration = parse_duration(secondpart, relative=relative)
            startdate = parse_date(firstpart)

            #See if we need to upconvert to datetime to preserve resolution
            if secondpart.find(datetimedelimiter) != -1:
                return (startdate, datetime.combine(startdate, datetime.min.time()) + duration)
            else:
                return (startdate, startdate + duration)
    else:
        #/
        if firstpart.find(datetimedelimiter) != -1 and secondpart.find(datetimedelimiter) != -1:
            #Both parts are datetimes
            return (parse_datetime(firstpart, delimiter=datetimedelimiter), parse_datetime(secondpart, delimiter=datetimedelimiter))
        elif firstpart.find(datetimedelimiter) != -1 and secondpart.find(datetimedelimiter) == -1:
            #First part is a datetime, second part is a date
            return (parse_datetime(firstpart, delimiter=datetimedelimiter), parse_date(secondpart))
        elif firstpart.find(datetimedelimiter) == -1 and secondpart.find(datetimedelimiter) != -1: